1. Field of the Invention
The present invention relates to a novel gas sensor and, more particularly, to a gas sensor which is suitable for detecting and controlling the oxygen concentration or the humidity in a gas.
2. Description of the Prior Art
Recently, there has been desired the development of a sensor for detecting the oxygen concentration or the humidity in the atmosphere or in the exhaust gas of an automobile or the like, especially, a safety device for (1) controlling the excess air ratio of a burning device such as a gas burner or a petroleum burner, (2) preventing the imcomplete combustion and the misfire, and (3) a highly reliable and inexpensive oxygen concentration detecting sensor which can be applied for the control or the like of the excess air ratio by detecting the oxygen concentration in the exhaust gas of the automobile.
As the sensor of this type, there has been proposed according to the prior art an oxygen concentration cell system which uses stabilized zirconia, ceria or the like. However, this system has difficulties in the design and installation of the structure required for the system has a defect that it cannot be stably operated at a temperature equal to or lower than 350.degree. C. This defect is eliminated by the development of a sensor which is made of an oxide having a perovskite structure and which makes use of the change in the resistance due to the difference in the oxygen concentration. The perovskite structure oxide is a crystal of such a double oxide that is expressed by a general formula: ABO.sub.3. However, the characteristics of a sensor of this type are greatly determined by the composition of the oxide and are largely influenced by the presence of a minute quantity of an impurity. Therefore, the base of the sensor of this type according to a prior art is prepared by a sintering process or by the calcining process wherein a mixture of the oxide with a vitreous substance is used so that the base may not be contaminated during the preparing process.
The sintering process is a method (references should be made to U.S. Pat. No. 4,044,601), by which material powders are formed into the shape of a sensor base or formed into slurry and applied to an electrically insulating base before the powders are sintered. According to this sintering process, a bulk having a certain size is prepared and is cut into predetermined thin and small pieces. Generally speaking, however, an oxide having the perovskite structure is so brittle because of insufficient sintering properties that the sensor using this structure has a low speed of response and has short durability. Moreover, in the sintering process it is difficult to form fine cracks in the structure which can enlarge the apparent effective surface area and consequently the sensor fails to have a high speed of response.
On the other hand, the calcining process using glass powders is a method (reference should be made to Japanese Patent Laid-Open Publication No. 56-31631), by which a mixture of the perovskite type oxide of material powders with glass powders is prepared into slurry and formed into a predetermined shape or is applied to a base before it is calcined. This calcining process has the problems that the characteristics of the product are dispersed over a wide range as a result of the mixture containing an impurity and that yield the characteristics of the product after a long use are deteriorated. Moreofer, since the surface is made so remarkably smooth by adding the glass powders that the apparent surface area thereof is reduced, the speed of response is lowered similarly to the product of the sintering process.